The present invention relates to a process for the reprocessing of waste paper, particularly printed papers which are difficult to recycle.
Due to our awareness of environmental matters, from the throw-away society of the 1960s, a trend towards extensive recycling has taken place. As far as paper usage is concerned, which has increased considerably in recent years, this means that waste paper as a raw material is no longer used only for packaging materials and hygiene products, but is becoming increasingly more important for high-quality varieties of paper. Because of the increased return of secondary fibers into the circulation of paper, some of the cellulose pulp can be saved. In this way, the environmental burden due to the chlorine bleach associated with the manufacture of cellulose pulp is also reduced.
The use of secondary fibers for high-quality papers requires the removal of impurities some of which are brought into the system as auxiliary products, to the greatest possible extent. In order to obtain a fiber stock with a high degree of whiteness, the print particles must be released from the fiber and removed. In the course of this, bleaching and de-inking processes further facilitate the improvement of the whiteness.
In the case of conventional printed matter, i.e., newspapers and magazines, printed by letterpress, gravure, or planographic methods, defibering in alkaline conditions has proven successful. In this defibering process, the separation of the fibers from the ink film is facilitated by use of mechanical forces. The addition of alkali causes, in addition to a partial saponification of the binding agent, swelling of the fibers, and thus the forcible release of the printing ink.
The removal of the printing ink from this fiber stock takes place in a second stage, where, basically, two different processes can be used, i.e., a washing process or a flotation process.
In Europe, the flotation process is used almost exclusively, while in the USA and Canada the washing process still predominates, although the trend is in the direction of flotation.
The addition of the chemicals takes place in both processes at the disintegration stage. By virtue of their good wetting capability, surfactants ease the separation of the ink particles and pigment particles from the fiber.
In order to prevent re-agglomeration of the released particles, in de-inking by washing, a dispersant is also added. The small particles of dirt remain finely divided and retain their hydrophilic character, so that they can be washed out of the fiber stock. For an optimum washing result, the dimensions of the particles should be below 15 .mu.m since larger particles are held back by the fiber mat.
Fillers and fines are also washed out along with the dirt particles, so that in comparison with flotation de-inking, the total yield of secondary fibers is lower in the washing process.
The flotation process operates with a significantly lower consumption of water. As early as the pulping stage, a bleaching agent, usually a peroxide compound, is added in order to prevent yellowing of the fiber. In order that the peroxide compound may develop its maximum activity, it is necessary to complex heavy metals, which catalyze the breakdown of the peroxide into cleavage products having no bleaching activity. Waterglass, which because of its complexing and dispersing effect is itself very suitable for the loosening process, but leads, however, to ever increasing difficulties in the system, is being replaced more and more by organic complexing agents. Fatty acids function as collecting agents for the printing ink particles and, together with the calcium ions present in the water, form hydrophobic soaps. The dirt agglomerates are taken up by the air bubbles, swim to the surface and are separated out of the system as black foam.
Papers produced by modern printing processes such as for example, flexographic printing, xerography, and related processes cannot be recycled by the above described flotation process. The particles of carbon black and pigment, which in the case of conventional papers after defibering are still in the form of relatively large agglomerates embedded in binding material, are present in flexographic inks in their original dimensions of a only few microns, i.e., carbon black of about 0.02 to about 0.03 .mu.m, and colored pigment of about 0.1 to about 0.4 .mu.m.
These small dirt particles would be acceptable for removal by the washing process but are not suited to the flotation process. A further difficulty with the flexographic inks is that the acrylate-based binding agent, functions as a dispersant in alkaline medium and distributes the ink particles in the fiber suspension in the finest form.
In the case of xerographic papers, the binder consists of polymer systems which harden as a result of chemical reactions during the printing process and fix the ink film firmly onto the fibers. The resulting ink film is very elastic and cannot be separated from the fiber during the loosening process. The prepared stock, therefore, still contains many dirt specks.
The improvement in the ability to de-ink these special printing papers has already been the subject of some patents, which, however, have been unable to establish themselves in practice; see, for example, U.S. Pat. No. 4,276,118, U.S. Pat. No. 3,635,789, U.S. Pat. No. 3,846,227, and U.S. Pat. No. 4,561,933.